How are Fossils Formed

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In the popular imagination fossils evoke images of huge dinosaur relics encased in stone but a fossil can be anything from micro organisms (the most common type of fossil by far), to foliage, trees, insects and animals of all sizes. Other than preserved animal remains there are also fossils of animal activity known as trace fossils. These can include footprints, track ways, burrows, bites and droppings (coprolites).

For about 3000 million years the only life on Earth was to be found in the sea, and it was not until 430 million years ago that plants and animals appeared on land. For this reason the oldest fossil types are marine creatures. Considering all the organisms that have lived within this huge time span fossil creation is very rare, relying on specific conditions to prevent the natural breakdown of organic remains and the process of the fossil to form.

After death the remains of most organisms last for only a relatively short period due to decomposition, scavenging by other animals and the effects of weathering, so the conditions that allow fossil formation need to be present very soon after the organism dies.


The most common process of fossilization is known as
permineralization and occurs when the organism dies and is covered by water. Ideal sources of water are a river bed or a delta. The water protects the animal from the elements and may contribute to the decomposition of the soft tissue by bacteria, leaving just the skeleton or shell.

Sediments in the water now bury the remains. The sooner this happens the more likely a fossil will form. Mud slides at a river delta can provide a quick build up of sediment and further protection from decomposition. As the layers of sediment continue to build up they eventually compress the lower levels into rock (a process known as lithification) which will encase the fossil.

The type of sediment the specimen is buried in will affect the quality of the fossil. For instance, if the sediment is sandy the fossil may have poor detail, but fine sediment will give the fossil much greater resolution.

The chemical composition of the sediment will also be influential. Iron in the sediment can give the fossil a red appearance, whilst phosphorous can darken it to grey or black.

If the water is especially rich in minerals they will percolate through the layers of lithified sediment giving an even greater chance for the fossil to form. Many types of organic material such as bone, wood and shell have empty spaces, filled with gas or liquid during life. These small spaces, even at the cellular level can become filled with mineral deposits as they percolate through the sediment and can result in a very finely detailed fossil.

The minerals can dissolve the organic material including bone, eventually replacing it and leaving a fossil that is an exact match of the original specimen. Mineral deposits can also stick to the sedimentary particles bonding them together to form a solid mass. These mineral processes can together create a fossil that provides a lot of information about the structure of blood vessels, feathers and brain size of the animal in life, and therefore give clues to its behaviour and natural environment.

Cast Fossils

A fossil formed where minerals replace the structure of the specimen is called a body fossil, distinguishing it from a cast fossil. A cast fossil occurs when none of the original specimen survives but leaves a mould'
of its shape in the sediment. The mould is a natural cast and fills with minerals making an external fossil impression of the organism.

An internal cast occurs when minerals fill an internal space within the animal, such as a shell cavity. It is possible for an external cast to later surround an internal cast and then also become a fossil or just fill with sediment.

Although permineralization and its variants account for a large proportion of fossils, it is not the only method for preserving organic remains. There a number of ways a fossil can occur and all involve different processes.

Asphalt Tar Pits

Hancock Park, otherwise known as La Brea Tar Pits is a 23 acre park in downtown Los
Angeles that contains over 100 pits filled with asphalt tar. The asphalt is an excellent preservative, and bones, teeth, shells, the exoskeletons of insects as well as plant seeds from the Pleistocene era (between ten and forty thousand years ago), have all been pulled from the pits. The pits were formed as crude oil seeped through natural crevices in the earth. The lighter elements of the oil then evaporated leaving a sticky tar.


Amber is another well known preservative with plenty of examples of spiders, insects and small lizards encased within it. Some trees exude a sticky sap to protect them from fungal infection which can engulf any insect or small animal that gets in its way. When the tree dies it falls into the swampy water in which it grew and eventually turns into coal. The sap is polymerised along with the trapped insect and hardens into amber. Rising sea levels submerge the coal layer which then becomes buried under the sea floor. The actions of the currents finally uncover the coal and begin to erode it allowing the amber, which is lighter than sea water to eventually float to the surface and drift to shore.


The most effective means of creating a fossil is by freezing so long as the specimen remains frozen from death until its discovery. There have been numerous remarkable finds in Alaska
and Siberia of mammoths and woolly Rhinoceroses. Some have been found with perfectly preserved stomach contents as well as food still in the mouth,

which suggests they were flash frozen.

There have also been cases of frozen human remains. One notable example is the body of a Bronze Age man discovered in the Alps
on the German and Italian border. The man was nicknamed Oetzi by his discoverers. Whether Oetzi can be regarded as a true fossil however depends on whether you accept the arbitrary date of ten thousand years to fall within the definition of being a fossil.


Recrystalization occurs in a fossil when the original skeletal material is present but has altered into a different crystal formation. This can occur at such small scales that even microstructural
details within the fossil can be observed.


A compression fossil is usually the chemical reduction of plant matter. The original organic matter remains but has undergone change to a thin carbon based layer.


This is an unusual type of fossil in which one organism is preserved with the skeletal remains of another. An example of this might be the shell of an animal that overgrows a smaller organism living on its shell.

Bog Bodies

This is another unusual type of fossil formation. Throughout northern Europe, Ireland
and the UK a number of examples of well preserved human remains dating back to pre-Roman Iron Age have been discovered in peat bogs. Unlike most other fossil types however, the acidity of the peat dissolves the skeletons of the bodies but preserves the soft tissue, which essentially becomes tanned like cowhide. Like the Oezti example however, due to the age of the remains it is open to question whether these bodies can be regarded as a true fossil type.

Although the process of fossil formation is extremely rare there are many ways in which organic remains can become naturally preserved. Whilst permineralization accounts for most fossil examples, this is by no means the only method a fossil can be created.

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